This invention relates to a batch process to prepare 4,4'-dihydroxydiphenyl sulfone from phenol and concentrated sulfuric acid and, more particularly, to a process for making a dihydroxydiphenyl product sulfone with a 4,4'-dihydroxydiphenyl sulfone isomer purity of at least 90 wt. % by reacting phenol and concentrated sulfuric acid employed in a mol ratio above 2 to 1 in a solvent mixture containing a lower-boiling and a higher-boiling hydrocarbon, removal by azeotropic distillation of water, hydrocarbon and phenol, phase separation of the distillate, and return of at least the phenol to the reaction at a rate which maintains a phenol to sulfuric acid mol ratio of at least 2 to 1.
The literature teaches that production of 4,4'-dihydroxydiphenyl sulfone generally results in crude reaction mixtures containing the desired 4,4'-dihydroxydiphenyl sulfone mixed with substantial amounts of by-product 2,4'-dihydroxydiphenyl sulfone and colored impurities. Also, that the 4,4'-isomer can be produced, but with difficulty, in an isomer yield of about 90 wt. % of the crude product, which product can be produced in about a 90 wt. % yield based upon starting material. This can be the case, for example, when the desired 4,4'-isomer is produced from starting materials comprising phenol and concentrated sulfuric acid, or phenol and p-phenolsulfonic acid. The production of a high percentage of 4,4'-isomer in the crude product and the separation of the 4,4'-isomer from admixed 2,4'-isomer heretofore has presented considerable difficulties and generally cannot be carried out in large scale operations by inexpensive preparative and separation means. For example, isomer separation can be accomplished by expensive multiple recrystallizations to obtain a high purity (99.5+) 4,4'-material. Procedures to yield a crude product containing a low percentage of the 2,2'-isomer are also complicated and costly.
It is known to prepare 4,4'-dihydroxydiphenyl sulfone by reaction of phenol and sulfuric acid (the proportion of phenol being in excess of the stoichiometric proportion of 2 mols of phenol per mol of sulfuric acid), but the desired product is usually accompanied by an unacceptably high proportion of the 2,4'-dihydroxydiphenyl sulfone isomer. In the past, various methods have been used to increase the 4,4'-isomer in the crude product as it is generally believed less expensive to produce a low amount of the 2,4'-isomer in the crude reaction product then remove it during purification of the crude product.
Japanese Patent Publication No. 51-98239 teaches reacting phenol and sulfuric acid in the presence of a solvent in which 4,4'-dihydroxydiphenyl sulfone is less soluble than the 2,4'-dihydroxydiphenyl sulfone. The solvent is gradually evaporated from the reaction system causing the 4,4'-dihydroxy isomer to separate and, since the two isomers are in equilibrium, isomerization of the 2,4'-dihydroxy isomer to the 4,4'-isomer proceeds. This method does, however, require careful control of the reaction conditions to ensure an appropriate balance between the rate of removal of the solvent and the rate of isomerization. Furthermore, the reaction product, after removal of the solvent, may be in the form of an intractable solid mass.
British Patent Application No. 2,030,566 discloses a process for the preparation of 4,4'-dihydroxydiphenyl sulfone wherein the recovered yield of 4,4'-dihydroxydiphenyl sulfone is increased by heating a mixture containing 4,4'-dihydroxydiphenyl sulfone and 2,4'-dihydroxydiphenyl sulfone in the presence of an acidic catalyst and an organic solvent. The solvent utilized in the process disclosed is o-dichlorobenzene and the catalysts utilized are organic acids or aqueous mineral acids. Dihydroxydiphenyl sulfone yields based upon starting materials from chlorinated solvents are not generally as good because the material is somewhat soluble in chlorinated solvents.
German Patent No. 2,708,388 discloses a process for preparing 4,4'-dihydroxydiphenyl sulfone by reaction of phenol with sulfuric acid in the presence of a solvent, o-dichlorobenzene, which forms an azeotrope with the water formed in the reaction. The water of reaction is removed by azeotropic distillation and the solvent is removed by distillation but the product is left as an intractable mass. The yield of the product based upon starting materials obtained by the process described is said to be 91.2% and contain less than 0.8% of the 2,4'-isomer. Because of the known solubility of the product in the solvent, special procedures have to be used to obtain a yield that high.
The presence of the by-product 2,4'-isomer, even in relatively small amounts in the purified product, greatly restricts the field of the application of the 4,4'-dihydroxydiphenyl sulfone product, particularly for polymer use. Therefore, it is of interest to reduce the amount of 2,4'-isomer in the crude product either in its preparation, purification, or both.
Another impurity made in the usual preparation processes but not often described in the literature is triphenol disulfone. This is the reaction product of three mols of phenol and two mols of sulfuric acid and is sometimes called trimer. Colored impurities are produced as well particularly in solventless processes.
One method of separating 4,4'-dihydroxydiphenyl sulfone from the 2,4'-isomer is taught in U.S. Pat. No. 3,065,274. The method of separation disclosed conducts the reaction in the presence of sym-tetrachloroethane solvent at the boiling temperature of the solvent, about 140.degree. C. at the pressure employed. This sym-tetrachloroethane is combined with the isomer mixture to be separated, the mixture cooled to a temperature in the range of about 80.degree. C to about 125.degree. C., preferably about 100.degree. C., and the solution filtered without lowering the temperature substantially, thereby obtaining a filter cake consisting of 4,4'-dihydroxydiphenyl sulfone free from substantial amounts of 2,4'-dihydroxydiphenyl sulfone. The filtrate consists essentially of sym-tetrachloroethane containing 2,4'-dihydroxydiphenyl sulfone in solution. This idea suffers from the fact that, if the system is cooled below 80.degree. C. during or prior to filtration, the 2,4'-isomer precipitates out of solution. In addition, the product obtained by this process apparently still contains several percent of the 2,4'-isomer based on the melting point given. Also, the sym-tetrachloroethane can react to a small extent with sulfuric acid, resulting in other types of impurities in the product.
European Patent Application 0293037 to AKZO N.V. teaches the preparation of the 4,4'-isomer by reaction of phenol with sulfuric acid in the presence of a suspending agent such as Isopar H, and an azeotroping agent such as Isopar E, wherein 2-3 mols of phenol per mol of sulfuric acid may be used, but a 1-10% excess of phenol is preferred. The crude product is said to contain greater than 95% 4,4'-isomer which it is alleged may be recrystallized in a single step to a product containing greater than 99.5% of the 4,4'-isomer. No examples of the recrystallization or any recrystallization solvents are given.
Now it has been found that, when two hydrocarbons such as Isopar H and E are used as a mixed solvent in the reaction of phenol and sulfuric acid and the reaction mixture azeotropically distilled, the vapor taken overhead when condensed can form three immiscible layers, the densest layer of which is mostly phenol with a smaller amount of water. Separation and return of this densest layer provides a convenient and economic way to return phenol to the reaction mixture to keep the phenol in the stoichiometric ratio or in slight excess during the reaction, as a proper phenol to sulfuric acid ratio is required for the preparation of a product having the optimal isomeric as well as colored product purity. Such a process conveniently makes a crude dihydroxydiphenol sulfone product in over 95 wt % with a 4,4'-isomer content greater than 90 wt %, a product which can be easily recrystallized from water to form a greater than 99 wt % 4,4'-isomer product.